Why do the workspace tabs disappear when I switch to another tab from Layout?
The problem is interface scaling. On my 4k monitor I had set the scale to 1.5. If I drop the scaling to 1.4 all the tabs are there for every workspace. Around 1.42 some workspaces have no tabs.

The 2/19/19 build (blender-2.80.0-git.855b3e68ef27-windows64) did not have the problem at 1.5 scale.

FYI